Composite materials with blend of thermoplastic particles

ABSTRACT

Pre-impregnated composite material (prepreg) is provided that can be cured to form composite parts that have high levels of damage tolerance. The matrix resin includes a thermoplastic particle component that is a blend of particles that have a melting point above the curing temperature and particles that have a melting point at or below the curing temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to pre-impregnated compositematerial (prepreg) that is used in making high performance compositeparts. More particularly, the invention is directed to providing prepregthat may be cured/molded to form composite parts having high strength,damage tolerance and interlaminar fracture toughness.

2. Description of Related Art

Composite materials are typically composed of a continuous resin matrixand reinforcing fibers as the two primary constituents. The compositematerials are often required to perform in demanding environments, suchas in the field of aerospace where the physical limits andcharacteristics of composite parts is of critical importance.

Pre-impregnated composite material (prepreg) is used widely in themanufacture of composite parts. Prepreg is a combination of an uncuredresin matrix and fiber reinforcement, which is in a form that is readyfor molding and curing into the final composite part. Bypre-impregnating the fiber reinforcement with resin, the manufacturercan carefully control the amount and location of resin that isimpregnated into the fiber network and ensure that the resin isdistributed in the network as desired. It is well known that therelative amount of fibers and resin in a composite part and thedistribution of resin within the fiber network have a large affect onthe structural properties of the part. Prepreg is a preferred materialfor use in manufacturing load-bearing structural parts and particularlyaerospace composite parts, such as wings, fuselages, bulkheads andcontrol surfaces. It is important that these parts have sufficientstrength, damage tolerance and other requirements that are routinelyestablished for such parts.

The fiber reinforcements that are commonly used in aerospace prepreg aremultidirectional woven fabrics or unidirectional tape that containsfibers extending parallel to each other. The fibers are typically in theform of bundles of numerous individual fibers or filaments that arereferred to as a “tows”. The fibers or tows can also be chopped andrandomly oriented in the resin to form a non-woven mat. These variousfiber reinforcement configurations are impregnated with a carefullycontrolled amount of uncured resin. The resulting prepreg is typicallyplaced between protective layers and rolled up for storage or transportto the manufacturing facility.

Prepreg may also be in the form of short segments of choppedunidirectional tape that are randomly oriented to form a non-woven matof chopped unidirectional tape. This type of pre-preg is referred to asa “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg issimilar to the more traditional non-woven fiber mat prepreg, except thatshort lengths of chopped unidirectional tape (chips) are randomlyoriented in the mat rather than chopped fibers.

The tensile strength of a cured composite material is largely dictatedby the individual properties of the reinforcing fiber and matrix resin,as well as the interaction between these two components. In addition,the fiber-resin volume ratio is an important factor. Cured compositesthat are under tension tend to fail through a mechanism of accumulateddamage arising from multiple tensile breakages of the individual fiberfilaments located in the reinforcement tows. Once the stress levels inthe resin adjacent to the broken filament ends becomes too great, thewhole composite can fail. Therefore, fiber strength, the strength of thematrix, and the efficiency of stress dissipation in the vicinity ofbroken filament ends will contribute to the tensile strength of a curedcomposite material.

In many applications, it is desirable to maximize the tensile strengthproperty of the cured composite material. However, attempts to maximizetensile strength can often result in negative effects on other desirableproperties, such as the compression performance and damage tolerance ofthe composite structure. In addition, attempts to maximize tensilestrength can have unpredictable effects on the tack and out-life of theprepreg. The stickiness or tackiness of the uncured prepreg is commonlyreferred to as “tack”. The tack of uncured prepreg is an importantconsideration during lay up and molding operations. Prepreg with littleor no tack is difficult to form into laminates that can be molded toform structurally strong composite parts. Conversely, prepreg with toomuch tack can be difficult to handle and also difficult to place intothe mold. It is desirable that the prepreg have the right amount of tackto insure easy handling and good laminate/molding characteristics. Inany attempt to increase strength and/or damage tolerance of a givencured composite material, it is important that the tack of the uncuredprepreg remain within acceptable limits to insure suitable prepreghandling and molding.

The “out-life” of prepreg is the length of time that the prepreg may beexposed to ambient conditions before undergoing an unacceptable degreeof curing. The out-life of prepreg can vary widely depending upon avariety of factors, but is principally controlled by the resinformulation being used. The prepreg out-life must be sufficiently longto allow normal handling, lay up and molding operations to beaccomplished without the prepreg undergoing unacceptable levels ofcuring. In any attempt to increase strength and/or damage tolerance of agiven cured composite material, it is important that the out-life of theuncured prepreg remain as long as possible to allow sufficient time toprocess, handle and lay up the prepreg prior to curing.

The most common method of increasing composite tensile performance is tochange the surface of the fiber in order to weaken the strength of thebond between matrix and fiber. This can be achieved by reducing theamount of electro-oxidative surface treatment of the fiber aftergraphitization. Reducing the matrix fiber bond strength introduces amechanism for stress dissipation at the exposed filament ends byinterfacial de-bonding. This interfacial de-bonding provides an increasein the amount of tensile damage a composite part can withstand beforefailing in tension.

Alternatively, applying a coating or “size” to the fiber can lower theresin-fiber bond strength. This approach is well known in glass fibercomposites, but can also be applied to composites reinforced with carbonfibers. Using these strategies, it is possible to achieve significantincreases in tensile strength. However, the improvements are accompaniedby a decrease in properties, such as compression after impact (CAI)strength, which requires high bond strength between the resin matrix andfibers.

Another alternative approach is to use a lower modulus matrix. Having alow modulus resin reduces the level of stress that builds up in theimmediate vicinity of broken filaments. This is usually achieved byeither selecting resins with an intrinsically lower modulus (e.g.cyanate esters), or by incorporating an ingredient such as an elastomer(carboxy-terminated butadiene-acrylonitrile [CTBN], amine-terminatedbutadiene-acrylonitrile [ATBN] and the like). Combinations of thesevarious approaches are also known.

Selecting lower modulus resins can be effective in increasing compositetensile strength. However, this can result in a tendency to damagetolerance, which is typically measured by a decrease in compressiveproperties, such as compression after impact (CAI) strength and openhole compression (OHC) strength. Accordingly, it is very difficult toachieve a simultaneous increase in both the tensile strength and damagetolerance

Multiple layers of prepreg are commonly used to form composite partsthat have a laminated structure. Delamination of such composite parts isan important failure mode. Delamination occurs when two layers debondfrom each other. Important design limiting factors include both theenergy needed to initiate a delamination and the energy needed topropagate it. The initiation and growth of a delamination is oftendetermined by examining Mode I and Mode II fracture toughness. Fracturetoughness is usually measured using composite materials that have aunidirectional fiber orientation. The interlaminar fracture toughness ofa composite material is quantified using the G1c (Double CantileverBeam) and G2c (End Notch Flex) tests. In Mode I, the pre-crackedlaminate failure is governed by peel forces and in Mode II the crack ispropagated by shear forces. The G2c interlaminar fracture toughness isrelated to CAI. Prepreg materials that exhibit high damage tolerancealso have high CAI and G2c values.

A simple way to increase interlaminar fracture toughness has been toincrease the ductility of the matrix resin by introducing thermoplasticsheets as interleaves between layers of prepreg. However, this approachtends to yield stiff, tack-free materials that are difficult to use.Another approach has been to include a tough resin interlayer of about25 to 30 microns thickness between fiber layers. The prepreg productincludes a resin rich surface containing fine, tough thermoplasticparticles. For the interlayer-toughened material, even though theinitial value of Mode II fracture toughness is about four times as highas that of carbon fiber prepregs without interlayer, the fracturetoughness value decreases as the crack propagates and converges at a lowvalue, which is almost the same as that of the non-interleaved system.Ultimately, the average G2c values hit a ceiling as the crack moves fromthe very tough interlaminar (resin-rich) region of the composite to theless tough intralaminar (fiber) zone.

Although existing prepregs are well suited for their intended use inproviding composite parts that are strong and damage tolerant, therestill is a continuing need to provide prepreg that may be used to makecomposite parts that have even higher levels of strength (e.g. tensilestrength and compression strength), damage tolerance (CAI) andinterlaminar fracture toughness (G1c and G2c).

SUMMARY OF THE INVENTION

In accordance with the present invention, pre-impregnated compositematerial (prepreg) is provided that can be molded to form compositeparts that have high levels of strength, damage tolerance andinterlaminar fracture toughness. This is achieved without causing anysubstantial negative impact upon the physical or chemicalcharacteristics of the uncured prepreg or the cured composite part.

The pre-impregnated composite materials of the present invention arecomposed of reinforcing fibers and a matrix. The matrix includes a resincomponent made up of difunctional epoxy resin in combination with amultifunctional aromatic epoxy resin. The matrix further includes athermoplastic particle component, a thermoplastic toughening agent and acuring agent. As a feature of the present invention, the thermoplasticcomponent is composed of high melting thermoplastic particles and lowmelting thermoplastic particles. The low melting particles melt duringthe curing process to provide an increase in damage tolerance andinterlaminar toughness that cannot be achieved when the high or lowmelting thermoplastic particles are used alone.

The present invention also covers methods for making the prepreg andmethods for molding the prepreg into a wide variety of composite parts.The invention also covers the composite parts that are made using theimproved prepreg.

It has been found that the use of a blend of both high and low meltingthermoplastic particles in accordance with the present invention resultsin the formation of prepreg that may be molded to form composite partsthat have improved damage tolerance and interlaminar toughness incomparison to conventional systems.

Additionally, it has surprisingly been found that the benefits ofimproved damage tolerance and interlaminar toughness can be obtainedwithout substantially affecting the other desirable physical propertiesof the prepreg (e.g. tack and out-life) or the resultant cured compositematerial (e.g. matrix-fiber bonding, strength, stress dissipation,compression performance, and the like).

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The pre-impregnated composite materials (prepreg) of the presentinvention may be used as a replacement for existing prepreg that isbeing used to form composite parts in the aerospace industry and in anyother application where high structural strength and damage tolerance isrequired. The invention involves substituting the resin formulations ofthe present invention in place of existing resins that are being used tomake prepreg. Accordingly, the resin formulations of the presentinvention are suitable for use in any of the conventional prepregmanufacturing and curing processes.

The pre-impregnated composite materials of the present invention arecomposed of reinforcing fibers and an uncured matrix. The reinforcingfibers can be any of the conventional fiber configurations that are usedin the prepreg industry. However, the matrix is a departure fromconventional prepreg industry practice. The matrix includes aconventional resin component that is made up of difunctional epoxy resinin combination with at least one multifunctional aromatic epoxy resinwith a functionality greater than two. The matrix further includes athermoplastic particle component, a thermoplastic toughening agent and acuring agent. As will be discussed in detail below, a feature of thepresent invention is that the thermoplastic particle component includesboth high melting thermoplastic particles that have melting points abovethe cure temperature and low melting thermoplastic particles that havemelting points at or below the cure temperature).

It was discovered that the use of a blend of high and low meltingthermoplastic particles imparts greater damage tolerance (CAI) andinterlaminar toughenss to the composite material. This gives rise to anincrease in the compression after impact (CAI) performance andinterlaminar toughness (G1c and G2c). The matrix resins of the presentinvention also impart very high tensile strength (e.g. open hole tensilestrength—OHT) to the composite material.

The difunctional epoxy resin used to form the resin component of thematrix may be any suitable difunctional epoxy resin. It will beunderstood that this includes any suitable epoxy resins having two epoxyfunctional groups. The difunctional epoxy resin may be saturated,unsaturated, cylcoaliphatic, alicyclic or heterocyclic. The resincomponent should make up from 40 to 65 weight percent of the matrix.

Difunctional epoxy resins, by way of example, include those based on:diglycidyl ether of Bisphenol F, Bisphenol A (optionally brominated),glycidyl ethers of phenol-aldelyde adducts, glycidyl ethers of aliphaticdiols, diglycidyl ether, diethylene glycol diglycidyl ether, Epikote,Epon, aromatic epoxy resins, epoxidised olefins, brominated resins,aromatic glycidyl amines, heterocyclic glycidyl imidines and amides,glycidyl ethers, fluorinated epoxy resins, or any combination thereof.The difunctional epoxy resin is preferably selected from diglycidylether of Bisphenol F, diglycidyl ether of Bisphenol A, diglycidyldihydroxy naphthalene, or any combination thereof. Most preferred isdiglycidyl ether of Bisphenol F. Diglycidyl ether of Bisphenol F isavailable commercially from Huntsman Advanced Materials (Brewster, N.Y.)under the trade names Araldite GY281 and GY285. A difunctional epoxyresin may be used alone or in any suitable combination with otherdifunctional epoxies.

The difunctional epoxy resin is present in the range 10 wt % to 40 wt %of the matrix resin. Preferably, the difunctional epoxy resin is presentin the range 15 wt % to 25 wt %. More preferably, the difunctional epoxyresin is present in the range 15 wt % to 20 wt %.

The second component of the matrix is one or more epoxy resins with afunctionality greater than two. It is preferred that at least one of themultifunctional epoxies has at least one meta-substituted phenyl ring inits backbone. Preferred multifunctional epoxy resins are those that aretrifunctional or tetrafunctional. Most preferably, the multifunctionalepoxy resin will be a combination of trifunctional and multifunctionalepoxies. The multifunctional epoxy resins may be saturated, unsaturated,cylcoaliphatic, alicyclic or heterocyclic.

Suitable multifunctional epoxy resins, by way of example, include thosebased upon: phenol and cresol epoxy novolacs, glycidyl ethers ofphenolaldelyde adducts; glycidyl ethers of dialiphatic diols; diglycidylether; diethylene glycol diglycidyl ether; aromatic epoxy resins;dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers;epoxidised olefins; brominated resins; aromatic glycidyl amines;heterocyclic glycidyl imidines and amides; glycidyl ethers; fluorinatedepoxy resins or any combination thereof.

A trifunctional epoxy resin will be understood as having the three epoxygroups substituted either directly or indirectly in a para or metaorientation on the phenyl ring in the backbone of the compound. Asmentioned previously, the meta orientation is preferred. Atetrafunctional epoxy resin will be understood as having the four epoxygroups substituted either directly or indirectly in a meta or paraorientation on the phenyl ring in the backbone of the compound.

It is also envisaged that the phenyl ring may additionally besubstituted with other suitable non-epoxy substituent groups. Suitablesubstituent groups, by way of example, include hydrogen, hydroxyl,alkyl, alkenyl, alkynyl, alkoxyl, aryl, aryloxyl, aralkyloxyl, aralkyl,halo, nitro, or cyano radicals. Suitable non-epoxy substituent groupsmay be bonded to the phenyl ring at the para or ortho positions, orbonded at a meta position not occupied by an epoxy group. Suitabletetrafunctional epoxy resins includeN,N,N′,N′-tetraglycidyl-m-xylenediamine (available commercially fromMitsubishi Gas Chemical Company (Chiyoda-Ku, Tokyo, Japan) under thename Tetrad-X), and Erisys GA-240 (from CVC Chemicals, Morrestown,N.J.). Suitable trifunctional epoxy resins, by way of example, includethose based upon: phenol and cresol epoxy novolacs; glycidyl ethers ofphenolaldelyde adducts; aromatic epoxy resins; dialiphatic triglycidylethers; aliphatic polyglycidyl ethers; epoxidised olefins; brominatedresins, aromatic glycidyl amines and glycidyl ethers; heterocyclicglycidyl imidines and amides; glycidyl ethers; fluorinated epoxy resinsor any combination thereof.

A preferred trifunctional epoxy resin is triglycidyl meta-aminophenol.Triglycidyl meta-aminophenol is available commercially from HuntsmanAdvanced Materials (Monthey, Switzerland) under the trade name AralditeMY0600, and from Sumitomo Chemical Co. (Osaka, Japan) under the tradename ELM-120.

Additional examples of suitable multifunctional epoxy resin include, byway of example, N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenyl methane(TGDDM available commercially as Araldite MY720 and MY721 from HuntsmanAdvanced Materials (Monthey, Switzerland), or ELM 434 from Sumitomo),triglycidyl ether of para aminophenol (available commercially asAraldite MY 0500 or MY 0510 from Huntsman Advanced Materials),dicyclopentadiene based epoxy resins such as Tactix 556 (availablecommercially from Huntsman Advanced Materials), tris-(hydroxyl phenyl),and methane-based epoxy resin such as Tactix 742 (available commerciallyfrom Huntsman Advanced Materials). Other suitable multifunctional epoxyresins include DEN 438 (from Dow Chemicals, Midland, Mich.), DEN 439(from Dow Chemicals), Araldite ECN 1273 (from Huntsman AdvancedMaterials), and Araldite ECN 1299 (from Huntsman Advanced Materials).

Epoxy resins with a functionality greater than two are present in therange 25 wt % to 45 wt % of the resin matrix. Preferably, a mixture isprovided that includes from 20 to 30 wt % trifunctional epoxy resins andfrom 5 to 15 wt % tetrafunctional epoxy.

The prepreg matrix in accordance with the present invention alsoincludes a thermoplastic particle component that is composed ofthermoplastic particles that have a melting point above the intendedcuring temperature and thermoplastic particles that have a melting pointat or below the intended curing temperature. Curing temperatures forepoxy resins in accordance with the present invention are typicallybetween 140° C. and 200° C. Preferred curing temperatures are in therange of 160° C. to 190° C. with curing temperatures of between about175° C. and 185° C. being particularly preferred. The particles thathave a high melting point (i.e. above the curing temperature) arereferred to herein as “high melting” thermoplastic particles. Theparticles that have a low melting point (i.e. at or below the curingtemperature) are referred to herein as “low melting” thermoplasticparticles. It is preferred that the high melting particles have amelting point that is at least 10° C. higher than the cure temperature.It is preferred that the low melting particles have a melting point thatis below the curing temperature and more preferably at least 10° C.below the cure temperature.

The thermoplastic component should include from 20 to 80 wt % highmelting thermoplastic particles and from 20 to 80 wt % low meltingthermoplastic particles. Preferably, the thermoplastic component willinclude from 40 to 60 wt % high melting thermoplastic particles and from40 to 60 wt % low melting thermoplastic particles. More preferably, thehigh and low melting thermoplastic particles are present in equalamounts.

The thermoplastic particles are polymers, which can be in the form ofhomopolymers, copolymers, block copolymers, graft copolymers, orterpolymers. The thermoplastic particles may be thermoplastic resinshaving single or multiple bonds selected from carbon-carbon bonds,carbon-oxygen bonds, carbon-nitrogen bonds, silicon-oxygen bonds, andcarbon-sulphur bonds. One or more repeat units may be present in thepolymer which incorporate the following moieties into either the mainpolymer backbone or to side chains pendant to the main polymer backbone:amide moieties, imide moieties, ester moieties, ether moieties,carbonate moieties, urethane moieties, thioether moieties, sulphonemoieties and carbonyl moieties. The thermoplastic paritcles can alsohave a partially cross-linked structure. The particles may be eithercrystalline or amorphous or partially crystalline.

Suitable examples of thermoplastic particles include, by way of example,polyamides, polycarbonates, polyacetal, polyphenylene oxide,polyphenylene sulphide, polyarylates, polyethers, polyesters,polyimides, polyamidoimides, polyether imides, polysulphones,polyurethanes, polyether sulphones, and polyether ketones. Polyamidesare the preferred type of thermoplastic particles. However, otherparticles may be used provided that they can be provided in both a highmelting form and a low melting form.

The polyamide particles may be made from polyamide 6 (caprolactame—PA6),polyamide 12 (laurolactame—PA12), polyamide 11, polyurethane, polymethylmethacrylate, crosslinked polymethyl methacrylate, densifiedpolyethylene sulfone, or any combination thereof. Preferredthermoplastic particles are polyamide particles that have a meltingpoint of between about 140° C. and 240° C. The particles should haveparticle sizes of below 100 microns. It is preferred that the particlesrange in size from 5 to 60 microns and more preferably from 10 to 30microns. It is preferred that the average particle size be around 20microns. The particles should be substantially spherical. The particlescan be made by anionic polymerization in accordance with PCT applicationWO2006/051222, by coextrusion, precipitation polymerization, emulsionpolymerization or by cryogenic grinding. It is preferred that theparticles are made by direct polymerization rather than by grinding orprecipitation. Suitable polyamide particles that may be used as eitherhigh or low melting particles in accordance with the present inventionare available commercially from Arkema of France under the trade nameOrgasol.

The thermoplastic particle component is present in the range 6 wt % to20 wt % of the matrix. Preferably, there will be from 3 to 10 wt % highmelting thermoplastic particles and from 3 to 10 wt % low meltingthermoplastic particles. More preferred is a thermoplastic componentthat is composed of equal amounts of high and low melting particles andwhich constitutes from 9 to 14 wt % of the matrix. The amount of lowmelting particles in the thermoplastic particle component can vary from10 to 90 weight percent with the high melting particles correspondinglyvarying from 90 to 10 weight percent of the thermoplastic particlecomponent. A more preferred range is from 25 to 75 weight percent lowmelting particles with a corresponding variation in high meltingparticles from 75 to 25 weight percent.

Polyamide particles come in a variety of grades that have differentmelting temperatures depending upon the particular polyamide, degree ofcopolymerization and degree of crystallinity. Particles that containmostly polyamide 6 (PA6) typically have melting points of above 190° C.,which is well above typical epoxy prepreg curing temperatures.Accordingly, little if any dissolution of the PA6 particles occursduring cure. Orgasol 1002 D NAT1 (100% PA6 particles having a degree ofcrystallinity equal to 51%, a glass transition temperature (Tg) of 26°C., a density of 1.15 g/cm³ (ISO 1183), a molecular weight of 60,200(g/mole)n, a solution viscosity of 0.93 with a melting point of 217° C.and an average particle size of 20 microns) is an example of highmelting polyamide particles. Another example of high melting polyamideparticles is Orgasol 3202 D Nat 1 which contains PA6/PA12 copolymerparticles (80% PA6 and 20% PA12) having a degree of crystallinity equalto 43%, a Tg of 29° C., a density of 1.09 g/cm³ (ISO 1183), a molecularweight of 60,800 (g/mole)n and a solution viscosity of 1.01. Thepolyamide copolymer particles in Orgasol 3202 D Nat 1 have an averageparticle size of 20 microns and a melting point of 194° C. The amount ofPA12 in the copolymer may be increased above 20%, if desired, providedthat the melting point of the particles does not drop below the curetemperature for the matrix and preferably is at least 10° C. above thecure temperature.

Polyamide 12 (PA12) particles and copolymers of PA6 and PA12 that haveless than about 70% PA6 have melting temperatures that are below thetypical curing temperatures for epoxy prepregs. These types of lowmelting particles undergo substantial melting at cure temperatures andare reformed into particles as the cured composite is cooled. Preferredlow melting polyamide particles are copolymers of PA6 and PA12. Forexample Orgasol 3502 D Nat 1 is a copolymer of 50% PA12 and 50% PA 6(Degree of crystallinity—26%, Tg—26° C., density—1.07 g/cm³ (ISO 1183),Molecular Weight—68,200 (g/mole)n, solution viscosity of 1.00) that hasa melting point of 142° C. with particle sizes averaging around 20microns. Further examples of suitable low melting polyamide particlesinclude: 1) Orgasol 3803 DNAT1, which is a copolymer of 80% PA12 and 20%PA6 (Molecular Weight—approx. 54,000 (g/mole)n, solution viscosity of1.0, that has a melting point of 160° C. with the mean particle sizebeing from 17 to 24 microns with there being less than 5% of theparticles with diameters less than 10 microns and less than 10% of theparticles with diameters of greater than 30 microns; and 2) developmentgrade Orgasol CG199, which is the same as Orgasol 3803 DNAT1, exceptthat the molecular weight of the copolymer is lower and the solutionviscosity is 0.71. The percentage of PA12 and PA6 in the low meltingpolyamide copolymer particles may be varied above and below thepercentages shown for Orgasol 3502 D Nat 1 and Orgasol 3803 D Nat 1provided that the melting point of the particles remains at or below thecure temperature for the matrix and preferably at least 10° C. below thecure temperature. Some polyamide particles may function as either low orhigh melting particles depending upon the curing temperature that isselected for the prepreg. For Example, Orgasol 3803 D Nat 1 particlesare considered to be high melting particles when the selected curingtemperature is below 160° C. However, if the selected curing temperatureis at or above 160° C., the Orgasol 3803 D Nat 1 particles areconsidered to be low melting.

The prepreg matrix resin includes at least one curing agent. Suitablecuring agents are those which facilitate the curing of theepoxy-functional compounds of the invention and, particularly,facilitate the ring opening polymerization of such epoxy compounds. In aparticularly preferred embodiment, such curing agents include thosecompounds which polymerize with the epoxy-functional compound orcompounds, in the ring opening polymerization thereof. Two or more suchcuring agents may be used in combination.

Suitable curing agents include anhydrides, particularly polycarboxylicanhydrides, such as nadic anhydride (NA), methylnadic anhydride(MNA—available from Aldrich), phthalic anhydride, tetrahydrophthalicanhydride, hexahydrophthalic anhydride (HHPA—available from Anhydridesand Chemicals Inc., Newark, N.J.), methyltetrahydrophthalic anhydride(MTHPA—available from Anhydrides and Chemicals Inc.),methylhexahydrophthalic anhydride (MHHPA—available from Anhydrides andChemicals Inc.), endomethylenetetrahydrophthalic anhydride,hexachloroendomethylene-tetrahydrophthalic anhydride (ChlorenticAnhydride—available from Velsicol Chemical Corporation, Rosemont, Ill.),trimellitic anhydride, pyromellitic dianhydride, maleic anhydride(MA—available from Aldrich), succinic anhydride (SA), nonenylsuccinicanhydride, dodecenylsuccinic anhydride (DDSA—available from Anhydridesand Chemicals Inc.), polysebacic polyanhydride, and polyazelaicpolyanhydride.

Further suitable curing agents are the amines, including aromaticamines, e.g., 1,3-diaminobenzene, 1,4-diaminobenzene,4,4′-diamino-diphenylmethane, and the polyaminosulphones, such as4,4′-diaminodiphenyl sulphone (4,4′-DDS—available from Huntsman),4-aminophenyl sulphone, and 3,3′- diaminodiphenyl sulphone (3,3′-DDS).

Also, suitable curing agents may include polyols, such as ethyleneglycol (EG—available from Aldrich), poly(propylene glycol), andpoly(vinyl alcohol); and the phenol-formaldehyde resins, such as thephenol-formaldehyde resin having an average molecular weight of about550-650, the p-t-butylphenol-formaldehyde resin having an averagemolecular weight of about 600-700, and the p-n-octylphenol-formaldehyderesin, having an average molecular weight of about 1200-1400, thesebeing available as HRJ 2210, HRJ-2255, and SP-1068, respectively, fromSchenectady Chemicals, Inc., Schenectady, N.Y.). Further as tophenol-formaldehyde resins, a combination of CTU guanamine, andphenol-formaldehyde resin having a molecular weight of 398, which iscommercially available as CG-125 from Ajinomoto USA Inc. (Teaneck,N.J.), is also suitable.

Different commercially available compositions may be used as curingagents in the present invention. One such composition is AH-154, adicyandiamide type formulation, available from Ajinomoto USA Inc. Otherswhich are suitable include Ancamide 400, which is a mixture ofpolyamide, diethyltriamine, and triethylenetetraamine, Ancamide 506,which is a mixture of amidoamine, imidazoline, andtetraethylenepentaamine, and Ancamide 1284, which is a mixture of4,4′-methylenedianiline and 1,3-benzenediamine; these formulations areavailable from Pacific Anchor Chemical, Performance Chemical Division,Air Products and Chemicals, Inc., Allentown, Pa.

Additional suitable curing agents include imidazole(1,3-diaza-2,4-cyclopentadiene) available from Sigma Aldrich (St. Louis,Mo.), 2-ethyl-4-methylimidazole available from Sigma Aldrich, and borontrifluoride amine complexes, such as Anchor 1170, available from AirProducts & Chemicals, Inc.

Still additional suitable curing agents include3,9-bis(3-aminopropyl-2,4,8,10-tetroxaspiro[5.5]undecane, which iscommercially available as ATU, from Ajinomoto USA Inc., as well asaliphatic dihydrazide, which is commercially available as Ajicure UDH,also from Ajinomoto USA Inc., and mercapto-terminated polysulphide,which is commercially available as LP540, from Morton International,Inc., Chicago, Ill.

The curing agent (s) are selected such that they provide curing of theresin component of the composite material when combined therewith atsuitable temperatures. The amount of curing agent required to provideadequate curing of the resin component will vary depending upon a numberof factors including the type of resin being cured, the desired curingtemperature and curing time. Curing agents typically includecyanoguanidine, aromatic and aliphatic amines, acid anhydrides, LewisAcids, substituted ureas, imidazoles and hydrazines. The particularamount of curing agent required for each particular situation may bedetermined by well-established routine experimentation.

Exemplary preferred curing agents include 4,4′-diaminodiphenyl sulphone(4,4′-DDS) and 3,3′-diaminodiphenyl sulphone (3,3′-DDS), bothcommercially available from Huntsman. The curing agent is present in anamount that ranges from 5 wt % to 45 wt % of the uncured matrix.Preferably, the curing agent is present in an amount that ranges from 10wt % to 30 wt %. More preferably, the curing agent is present in therange 15 wt % to 25 wt % of the uncured matrix. Most preferred arematrix resins that contain from 16 wt % to 22 wt % curing agent.

4,4′-DDS is a preferred curing agent. It is preferably used as the solecuring agent in amounts ranging from 15 wt % to 25 wt %. The use ofsubstantial amounts of 3,3′-DDS as the curing is not preferred. It isexpected that the more reactive 3,3′-DDS will provide increased strengthin the neat cured resins, but that the resulting prepregs will have tackproperties that are not nearly as good as those using the less reactive4,4′-DDS. Accordingly, to achieve the optimum balance of prepregoutlife, tack and mechanical performance of the cured composite part, itis preferred that less reactive curing agents, such as 4,4′-DDS and thelike, be used at an amine to epoxy stoichiometry of about 70 to 80percent.

The matrix of the present invention also preferably includes athermoplastic toughening agent. Any suitable thermoplastic polymers maybe used as the toughening agent. Typically, the thermoplastic polymer isadded to the resin mix as particles that are dissolved in the resinmixture by heating prior to addition of the curing agent. Once thethermoplastic agent is substantially dissolved in the hot matrix resinprecursor (i.e. the blend of epoxy resins), the precursor is cooled andthe remaining ingredients (curing agent and insoluble thermoplasticparticles) are added.

Exemplary thermoplastic toughening agents/particles include any of thefollowing thermoplastics, either alone or in combination: polyamides,copolyamides, polyimides, aramids, polyketones, polyetheretherketones,polyesters, polyurethanes, polysulphones, polyethersulfones, highperformance hydrocarbon polymers, liquid crystal polymers, PTFE,elastomers, and segmented elastomers.

Toughening agent is present in the range 45 wt % to 5 wt % of theuncured resin matrix. Preferably, the toughening agent is present in therange 25 wt % to 5 wt %. More preferably, the toughening agent ispresent in the range 20 wt % to 10 wt %. Most preferably, the toughenagent is present in the range of 13 wt % to 17 wt % of the matrix resin.A suitable toughening agent, by way of example, are PES particles soldunder the tradename Sumikaexcel 5003P, which is commercially availablefrom Sumitomo Chemicals. Alternatives to 5003P are Solvaypolyethersulphone 105RP, or the non-hydroxyl terminated grades such asSolvay 1054P.

The matrix resin may also include additional ingredients, such asperformance enhancing or modifying agents and additional thermoplasticpolymers provided they do not adversely affect the tack and outlife ofthe prepreg or the strength and damage tolerance of the cured compositepart. The performance enhancing or modifying agents, for example, may beselected from flexibilizers, toughening agents/particles, accelerators,core shell rubbers, flame retardants, wetting agents, pigments/dyes, UVabsorbers, anti-fungal compounds, fillers, conducting particles, andviscosity modifiers. Suitable additional thermoplastic polymers for useas additional toughening agents include any of the following, eitheralone or in combination: polyether sulphone (PES), polyetherethersulphone (PEES), polyphenyl sulphone, polysulphone, polyimide,polyetherimide, aramid, polyamide, polyester, polyketone,polyetheretherketone (PEEK), polyurethane, polyurea, polyarylether,polyarylsulphides, polycarbonates, polyphenylene oxide (PPO) andmodified PPO.

Suitable accelerators are any of the urone compounds that have beencommonly used. Specific examples of accelerators, which may be usedalone or in combination, include N,N-dimethyl, N′-3,4-dichlorphenyl urea(Diuron), N′-3-chlorophenyl urea (Monuron), and preferablyN,N-(4-methyl-m-phenylene bis[N′,N′-dimethylurea] (e.g. Dyhard UR500available from Degussa).

Suitable fillers include, by way of example, any of the following eitheralone or in combination: silicas, aluminas, titania, glass, calciumcarbonate and calcium oxide.

Suitable conducting particles, by way of example, include any of thefollowing either alone or in combination: silver, gold, copper,aluminum, nickel, conducting grades of carbon, buckminsterfullerene,carbon nanotubes and carbon nanofibres. Metal-coated fillers may also beused, for example nickel coated carbon particles and silver coatedcopper particles.

The matrix resin may include, if desired, an additional non-epoxythermosetting polymeric resin. Once cured, a thermoset resin is notsuitable for melting and remolding. Suitable non-epoxy thermoset resinmaterials for the present invention include, but are not limited to,resins of phenol formaldehyde, urea-formaldehyde,1,3,5-triazine-2,4,6-triamine (Melamine), bismaleimide, vinyl esterresins, benzoxazine resins, phenolic resins, polyesters, cyanate esterresins, epoxide polymers, or any combination thereof. The thermosetresin is preferably selected from epoxide resins, cyanate ester resins,bismaleimide, vinyl ester, benzoxazine and phenolic resins. If desired,the matrix may include further suitable resins containing phenolicgroups, such as resorcinol based resins, and resins formed by cationicpolymerization, such as DCPD—phenol copolymers. Still additionalsuitable resins are melamine-formaldehyde resins, and urea-formaldehyderesins.

The resin matrix is made in accordance with standard preprep matrixprocessing. In general, the various epoxy resins are mixed together atroom temperature to form a resin mix to which the thermoplastictoughening agent is added. This mixture is then heated to a temperatureabove the melting point of the thermoplastic toughening agent for asufficient time to substantially melt the toughening agent. The mixtureis then cooled down to room temperature or below and the remainder ofthe ingredients (insoluble thermoplastic particles, curing agent andother additive, if any) are mixed into the resin to form the finalmatrix resin that is impregnated into the fiber reinforcement.

The matrix resin is applied to the fibrous reinforcement in accordancewith any of the known prepreg manufacturing techniques. The fibrousreinforcement may be fully or partially impregnated with the matrixresin. In an alternate embodiment, the matrix resin may be applied tothe fiber fibrous reinforcement as a separate layer, which is proximalto, and in contact with, the fibrous reinforcement, but does notsubstantially impregnate the fibrous reinforcement. The prepreg istypically covered on both side with a protective film and rolled up forstorage and shipment at temperatures that are typically kept well belowroom temperature to avoid premature curing. Any of the other prepregmanufacturing processes and storage/shipping systems may be used ifdesired.

The fibrous reinforcement of the prepreg may be selected from hybrid ormixed fiber systems that comprise synthetic or natural fibers, or acombination thereof. The fibrous reinforcement may preferably beselected from any suitable material such as fiberglass, carbon or aramid(aromatic polyamide) fibers. The fibrous reinforcement is preferablycarbon fibers.

The fibrous reinforcement may comprise cracked (i.e. stretch-broken) orselectively discontinuous fibers, or continuous fibers. It is envisagedthat use of cracked or selectively discontinuous fibers may facilitatelay-up of the composite material prior to being fully cured, and improveits capability of being shaped. The fibrous reinforcement may be in awoven, non-crimped, non-woven, unidirectional, or multi-axial textilestructure form, such as quasi-isotropic chopped prepreg. The woven formmay be selected from a plain, satin, or twill weave style. Thenon-crimped and multi-axial forms may have a number of plies and fiberorientations. Such styles and forms are well known in the compositereinforcement field, and are commercially available from a number ofcompanies, including Hexcel Reinforcements (Villeurbanne, France).

The prepreg may be in the form of continuous tapes, towpregs, webs, orchopped lengths (chopping and slitting operations may be carried out atany point after impregnation). The prepreg may be an adhesive orsurfacing film and may additionally have embedded carriers in variousforms both woven, knitted, and non-woven. The prepreg may be fully oronly partially impregnated, for example, to facilitate air removalduring curing.

An exemplary preferred matrix resin includes from 15 wt % to 20 wt %Bisphenol-F diglycidyl ether; from 20 wt % to 30 wt %triglycidyl-m-aminophenol (trifunctional epoxy resin); from 5 to 15 wt %tetrafunctional para-glycidyl amine; from 15 wt % to 25 wt %diaminodiphenylsulphone (primarily 4,4-DDS as a curing agent); from 3 wt% to 10 wt % high melting polyamide particles (Orgasol 1002D Nat 1);from 3 wt % to 10 wt % low melting polyamide particles (Orgasol 3803DNat 1), and from 10 wt % to 20 wt % poly(ether sulphone) as a tougheningagent.

The prepreg may be molded using any of the standard techniques used toform composite parts. Typically, one or more layers of prepreg are placein a suitable mold and cured to form the final composite part. Theprepreg of the invention may be fully or partially cured using anysuitable temperature, pressure, and time conditions known in the art.Typically, the prepreg will be cured in an autoclave at temperatures ofbetween 160° C. and 190° C. The composite material may more preferablybe cured using a method selected from UV-visible radiation, microwaveradiation, electron beam, gamma radiation, or other suitable thermal ornon-thermal radiation.

Composite parts made from the improved prepreg of the present inventionwill find application in making articles such as numerous primary andsecondary aerospace structures (wings, fuselages, bulkheads and thelike), but will also be useful in many other high performance compositeapplications including automotive, rail and marine applications wherehigh tensile strength, compressive strength, interlaminar fracturetoughness and resistance to impact damage are needed.

In order that the present invention may be more readily understood,reference will now be made to the following background information andexamples of the invention.

EXAMPLE 1

A preferred exemplary resin formulation in accordance with the presentinvention is set forth in TABLE 1. A matrix resin was prepared by mixingthe epoxy ingredients at room temperature with the polyethersulfone toform a resin blend that was heated to 130° C. for 60 minutes tocompletely dissolve the polyethersulfone. The mixture was cooled to 80°C. and the rest of the ingredients added and mixed in thoroughly.

TABLE 1 Amount (Wt %) Ingredient 17.30 Bisphenol-F diglycidyl ether(GY285) 26.15 Trifunctional meta-glycidyl amine (MY0600) 10.46Tetrafunctional para-glycidyl amine (MY721) 20.90 Aromatic diaminecurative (4,4-DDS) 15.69 Toughener (Sumikaexcel 5003P polyether sulfone)4.75 High melting polyamide particles (Orgasol 1002 D Nat 1) 4.75 Lowmelting polyamide particles (Orgasol 3803 D Nat 1)

Exemplary prepreg was prepared by impregnating one or more layers ofunidirectional carbon fibers with the resin formulation of TABLE 1. Theunidirectional carbon fibers were used to make a prepreg in which thematrix resin amounted to 35 weight percent of the total uncured prepregweight and the fiber areal weight was 190 grams per square meter (gsm).A variety of prepreg lay ups were prepared using standard prepregfabrication procedures. The prepregs were cured in an autoclave at 180°C. for about 2 hours. The cured prepregs were then subjected to standardtests to determine their tensile strength, tolerance to damageinterlaminar fracture toughness as described below.

In-plane shear modulus (IPM) was determined at room temperature using an4-ply laminate with configuration (45, −45, −45, 45). The laminate wascured for 2 hours at 180° C. in an autoclave and gave a nominalthickness of 0.75 mm. Consolidation was verified by C-scan. Thespecimens were cut and tested according to Boeing BMS 8-276 and citedBoeing methods. Results quoted are not normalized.

Compression after Impact (CAI) after a 270 in-lb impact was determinedusing a 24-ply quasi-isotropic laminate. The laminate was cured at 180°C. for 2 hours in the autoclave. The final laminate thickness was about4.5 mm. The consolidation was verified by c-scan. The specimens weremachined, impacted and tested in accordance with Boeing test methodBSS7260 per BMS 8-276. Values are normalized to a nominal cured laminatethickness of 0.18 inches.

Open hole compression (OHC) was determined at room temperature using a16-ply quasi-isotropic laminate. The laminate was cured for 2 hours at180° C. in an autoclave and gave a nominal thickness of 3 mm.Consolidation was verified by C-scan. The specimens were machined andtested in accordance with Boeing test method BMS BSS 7260 per BMS 8-726.Values are normalized to a nominal cured laminate thickness of 0.12inch.

Open hole tension (OHT) was determined at room temperature using a16-ply quasi-isotropic laminate. The laminate was cured for 2 hours at180° C. in an autoclave and gave a nominal thickness of 1.5 mm.Consolidation was verified by C-scan. The specimens were machined andtested in accordance with Boeing test method BSS 7260 per BMS 8-276.Values are normalized to a nominal cured laminate thickness of 0.06inch.

G1c and G2c are standard tests that provide a measure of theinterlaminar fracture toughness of the cured laminate. G1c and G2c weredetermined as follows. A 20-ply unidirectional laminate was cured with a3 inch fluoroethylene polymer (FEP) film inserted along one edge, at themidplane of the layup, perpendicular to the fiber direction to act as acrack starter. The laminate was cured for 2 hours at 180° C. in anautoclave and gave a nominal thickness of 3.8 mm. Consolidation wasverified by C-scan. Both G1c and G2c were machined from the same curedlaminate. G1c was tested in accordance with Boeing test method BSS7273and G2c was tested in accordance with BMS 8-276. Values for G1c and G2cwere not normalized.

The cured prepreg had an IPM of about 0.7 msi. The OHT was 72.2 ksi withthe OHC and CAI being 45.0 ksi and 55.7 ksi, respectively. G1c was 2.4in-lb/in² and G2c was 12.9 in-lb/in².

A second exemplary prepreg was made, cured and tested in the same manneras set forth above, except that the amounts of Orgasol 1002 and 3803were changed so that there was 75 wt % Orgasol 1002 in the thermoplasticparticle component and 25 wt % Orgasol 3803. This exemplary prepreg hadan OHC of 43.5 ksi and a CAI of 53.3 ksi. G1c was 2.2 in-lb/in² and G2cwas 11.6 in-lb/in².

A comparative prepregs (1C1 and 1C2) were made and tested in the samemanner as the above-described preferred exemplary prepreg. 1C1 and 1C2were identical to the exemplary prepreg, except that 13.5 (1C1) and 9.5(1C2) weight percent of the high melting Orgasol 1002 DNAT1 polyamide 6particles were used instead of a blend of high and low melting polyamideparticles. The resulting cured prepreg for 1C1 had an IPS modulus ofabout 0.70 msi. The OHT was 74.8 ksi with the OHC and CAI being 44.4 ksiand 47.4 ksi, respectively. G1c was 2.1 in-lb/in² and G2c was 6.7in-lb/in². For 1C2, OHC was 44.2 ksi and CAI was 52.8 ksi. G1c was 1.9in-lb/in² and G2c was 10.9 in-lb/in²

The above example demonstrates that an unexpected substantial increasein damage tolerance and interlaminar fracture toughness occurs when ablend of high and low melting polyamide particles are used in place ofhigh melting polyamide particles. In addition, this increase in bothinterlaminar fracture toughness and damage tolerance was achievedwithout adversely affecting the outlife and tack of the prepreg or theother physical/chemical properties of the cured part.

EXAMPLE 2

Additional exemplary prepregs (2a-2c) were prepared and cured in thesame manner as Example 1. These prepregs used different epoxy resinformulations in which the blends of high and low melting polyamideparticles were varied both in type and/or amount. The prepregs wereprepared using a different carbon fiber. The prepregs contained 35%resin by weight and had a fiber areal weight of 268 gsm. Theformulations used for these exemplary prepregs are set forth in TABLE 2.

TABLE 2 Component (wt %) 2a 2b 2c GY281 24.8 24.8 26.19 MY0600 28.0328.03 29.6 PES 5003P 15.0 15.0 15.0 4,4-DDS 18.66 18.66 19.7 Orgasol1002 6.75 6.75 4.75 DNAT1 Orgasol 3502 6.75 4.75 DNAT1 Orgasol 3803 6.75DNAT1

The cured prepregs were tested according to Airbus AITM methods. Theresults are set forth in TABLE 3.

TABLE 3 2a 2b 2c IPS (ksi) 14.9 16.8 17.0 IPM (msi) 0.73 0.73 0.73 OHT(ksi) 104.6 120.5 — OHC (ksi) 54.4 60.2 57.9 CAI (ksi) 47.3 49.3 48.3

The mechanical properties for the compositions of TABLE 2,which arespecified in TABLE 3 were determines as follows:

In-plane shear strength (IPS) and modulus (IPM) were determined at roomtemperature using an 8-ply laminate with configuration (45, −45, 45,−45). The laminate was cured for 2 hours at 180° C. in an autoclave andgave a nominal thickness of 2 mm. Consolidation was verified by C-scan.The specimens were cut and tested according to Airbus test method AITM1.0002. Results quoted are not normalized.

Compression after Impact (CAI) after a 270 in-lb impact was determinedusing a 16-ply quasi-isotropic laminate. The laminate was cured at 180°C. for 2 hours in the autoclave. The final laminate thickness was about4 mm. The consolidation was verified by c-scan. The specimens were cutand tested in accordance with Airbus test method AITM 1.0010 issue 2,June 1994. The results were normalized at 60% volume fraction based onnominal cure ply thickness as per EN 3784 method B.

Open hole compression (OHC) was determined at room temperature using a20 ply laminate with 40/40/20 lay-up. The laminate was cured for 2 hoursat 180° C. in an autoclave and gave a nominal thickness of 5 mm.Consolidation was verified by C-scan. The specimens were cut up andtested in accordance with Airbus test method AITM 1.0008. Results arevalues normalized to 60% volume fraction based on nominal cure plythickness with calculation carried out as per EN 3784 method B.

Open hole tension (OHT) was determined at room temperature using a20-ply with 40/40/20 lay-up. The laminate was cured for 2 hours at 180°C. in an autoclave and gave a nominal thickness of 5 mm. Consolidationwas verified by C-scan. The specimens were cut up and tested inaccordance with Airbus test method AITM 1.0008. The results are valuesthat were normalized to 60% volume fraction based on nominal cure plythickness with calculation carried out as per EN 3784 method B.

The Airbus standard AIMS 05-01-002, which cites many of the Airbus testmethods (AITM) used for generation of the data in this example, aresimilar to BMS 8-276, which sets forth the Boeing test methods forprimary structure composite materials. However, the Airbus test methodsare different and utilize different lay-up and sample dimensions.

Comparative examples 2C1-2C3 were prepared in the same manner as theexemplary prepregs except that a blend of high and low melting polyamideparticles was not used. The formulations for these comparative examplesare set forth in TABLE 4.

TABLE 4 Component (wt %) 2C1 2C2 2C3 GY281 24.8 24.8 24.8 MY0600 28.028.0 28.0 PES 5003P 15.0 15.0 15.0 4,4-DDS 18.7 18.7 18.7 Orgasol 100213.5 DNAT1 Orgasol 3502 13.5 DNAT1 Oragasol CG199 13.5

The cured comparative prepregs were subjected to the same testingprocedures as in Example 2a-2c. The results are set forth in TABLE 5.

TABLE 5 2C1 2C2 2C3 IPS (ksi) 14.9 14.5 10.2 IPM (msi) 0.80 0.73 0.62OHT (ksi) 119.4 118.1 155.2 OHC (ksi) 58.3 57.2 57.0 CAI (ksi) 41.5-42.544.7 35.2

This example demonstrates that an unexpected increase in CAI occurs whenblends of high and low melting polyamide particles are used. Thisincrease is achieved without negatively affecting the other propertiesof both the uncured and cured prepreg. As shown in TABLE 5, the CAI fornon-blended formulations of Oragasol 1002, 3502 and CG199 particles(2C1-2C3) are 41.5-42.5, 44.7 and 35.2 ksi, respectively. When theparticles are blended together, as in Examples 2a-2c, the resultingCAI's unexpectedly jump to 47.3-49.3 ksi. This is an unexpectedadvantage that cannot be achieved when using high or low meltingparticles by themselves.

EXAMPLE 3

Further exemplary (3) and comparative prepregs (3C1 and 3C2) were madein the same manner as Example 1. The same type of carbon fiber was usedas for Example 1. However, the fiber areal weight of the prepreg wasincreased to 268 gsm and the tetrafunctional para-glycidyl amine wasdropped from the resin formulation. The formulation for the resins usedin this example are set forth in TABLE 6.

TABLE 6 3 3C1 3C2 (Wt %) (Wt %) (Wt %) Ingredient 26.19 24.80 24.80Bisphenol-F diglycidyl ether (GY281) 29.60 28.03 28.03 Trifunctionalmeta-glycidyl amine (MY0600) 19.70 18.66 18.70 Aromatic diamine curative(4,4-DDS) 15.00 15.00 15.00 Toughener (Sumikaexcel 5003P polyethersulfone) 4.75 13.00 High melting polyamide particles (Orgasol 1002 DNat 1) 4.75 — Low melting polyamide particles (Orgasol 3803 D Nat 1)13.5 High melting polyamide particles (Orgasol 3202 D Nat 1)

The cured prepregs were subjected to the same testing procedures as inExamples 2a-2c. The results are set forth in TABLE 7.

TABLE 7 3 3C1 3C2 IPS (ksi) 18.9 20.0 — IPM (msi) 0.74 0.83 — OHT (ksi)100.5 98.2 — OHC (ksi) 53.7 52.7 52.9 CAI (ksi) 55.6 51.9 52.4

This example demonstrates that an unexpected increase in CAI occurs whenblends of high and low melting polyamide particles are used. Thisincrease is achieved without negatively affecting the other propertiesof both the uncured and cured prepreg. It should be noted that thedamage tolerance (CAI, OHT and OHC) of the exemplary prepreg (3) washigher than that of the comparative prepreg 3C1, even though there was 4weight % less polyamide particles present in the matrix resin.

EXAMPLE 4

Comparative prepregs (C4) were made in the same manner as Example 1. Thesame type of carbon fiber was used. However, the fiber areal weight ofthe prepreg was increased to 268 gsm and the trifunctional meta-glycidylamine was replaced with a trifunctional para-glycidyl amine. The resinformulation for these comparative prepregs is set forth in TABLE 8.

TABLE 8 Amount (Wt %) Ingredient 22.10 Bisphenol-F diglycidyl ether(GY285) 10.10 Trifunctional para-glycidyl amine (MY0510) 21.10Tetrafunctional para-glycidyl amine (MY721) 19.20 Aromatic diaminecurative (4,4-DDS) 14.00 Toughener (Sumikaexcel 5003P polyether sulfone)13.50 High melting polyamide particles (Orgasol 1002 D Nat 1) 0.00 Lowmelting polyamide particles (Orgasol 3803 D Nat 1)

The cured comparative prepregs (C4) were subjected to the same testingas in Examples 2a-2c. The cured prepreg (C4) had an IPS of about 15.2ksi and an IPM of about 0.75 msi. The OHT was 86.2 ksi with the OHC andCAI being 51.8 ksi and 45.5 ksi, respectively.

Exemplary prepreg in accordance with the present invention are made bychanging the types and amounts of high and low melting polyamideparticles in the thermoplastic particle component of the C4 resinformulation. Exemplary combinations include: 4a) 6.75 wt % Oragsol 1002D Nat 1 and 6.75 wt % Orgasol 3803 D Nat 1; 4b) 6.75 wt % Oragsol 1002 DNat 1 and 6.75 wt % Orgasol 3502 D Nat 1; 4c) 6.75 wt % Oragsol 3202 DNat 1 and 6.75 wt % Orgasol 3803 D Nat 1; and 4d) 6.75 wt % Oragsol 3202D Nat 1 and 6.75 wt % Orgasol 3502 D Nat 1.

All of the above exemplary prepregs 4a-4d, when subjected to the sametesting as in Examples 2a-2c will show increases in damage tolerance(CAI) when compared to the above results set forth in the comparativeexample (C4). These increases are achieved without negatively affectingthe other physical and chemical properties of the uncured and curedprepreg.

Other exemplary prepreg may be made by varying the relative amounts ofthe polyamide particles set forth in 4a-4b and by varying the totalamount of polyamide particles in the resin provided that the limitationsset forth previously in this detailed description are observed.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-describedembodiments, but is only limited by the following claims.

1. A pre-impregnated composite material for curing at a curingtemperature, said pre-impregnated composite material comprising: A)reinforcing fibers; and B) a matrix comprising: a) a resin componentcomprising a difunctional epoxy resin and at least one multifunctionalaromatic epoxy resin with a functionality greater than two; b) athermoplastic particle component comprising high melting thermoplasticparticles that have a melting point above said curing temperature andlow melting thermoplastic particles that have a melting point at orbelow said curing temperature; d) a thermoplastic toughening agent; ande) a curing agent.
 2. A pre-impregnated composite material according toclaim 1 wherein said curing temperature is between 140° C. and 200° C.3. A pre-impregnated composite material according to claim 1 whereinsaid high melting thermoplastic particles are selected from the groupconsisting of polyamide 6 and said low melting thermoplastic particlesare selected from the group consisting of copolymers of polyamide 6 andpolyamide
 12. 4. A pre-impregnated composite material according to claim1 wherein said reinforcing fibers are selected from the group consistingof glass, carbon and aramid.
 5. A pre-impregnated composite materialaccording to claim 1 wherein said difunctional epoxy resin is selectedfrom the group consisting of diglycidyl ether of bisphenol F, diglycidylether of bisphenol A, diglycidyl dihydroxy naphthalene and combinationsthereof.
 6. A pre-impregnated composite material according to claim 1wherein said resin component comprises a trifunctional meta-glycidylamine and a tetrafunctional para-glycidyl amine.
 7. A pre-impregnatedcomposite material according to claim 1 wherein the amount of said highmelting thermoplastic particles in said thermoplastic component is aboutequal to the amount of said low melting thermoplastic particles in saidthermoplastic component.
 8. A pre-impregnated composite materialaccording to claim 1 wherein said toughening agent is selected from thegroup consisting of polyether sulfone, polyether ethersulfone,polyphenyl sulphone, polysulfone, polyimide, polyetherimide, aramid,polyamide, polyester, polyketone, polyetheretherketone, polyurethane,polyurea, polyarylether, polyarylsulphide, polycarbonate andpolyphenylene oxide.
 9. A pre-impregnated composite material accordingto claim 1 wherein said curing agent is an aromatic amine.
 10. Apre-impregnated composite material according to claim 1 wherein saidmatrix comprises: 40 to 65 weight percent of said resin component; 3 to10 weight percent of said high melting thermoplastic particles; 3 to 10weight percent of said low melting thermoplastic particles; 10 to 20weight percent of said thermoplastic toughening agent; and 15 to 25weight percent of said curing agent.
 11. A pre-impregnated compositematerial according to claim 10 wherein said resin component comprises atrifunctional meta-glycidyl amine and a tetrafunctional para-glycidylamine.
 12. A pre-impregnated composite material according to claim 11wherein said difunctional epoxy resin is diglycidyl ether of bisphenolF.
 13. A pre-impregnated composite material according to claim 12wherein said high melting thermoplastic particles are selected from thegroup consisting of polyamide 6 and said low melting thermo plasticparticles are selected from the group consisting of copolymers ofpolyamide 6 and polyamide
 12. 14. A pre-impregnated composite materialaccording to claim 13 wherein said thermoplastic toughening agent ispolyether sulfone.
 15. A pre-impregnated composite material according toclaim 14 wherein said curing agent is 4,4-diaminodiphenylsulfone.
 16. Apre-impregnated composite material according to claim 13 wherein theamount of said high melting thermoplastic particles in saidthermoplastic component is about equal to the amount of and low meltingthermoplastic particles in said thermoplastic component.
 17. Apre-impregnated composite material according to claim 15 wherein saidmatrix comprises 15 to 20 weight percent of diglycidyl ether ofbisphenol F; 20 to 30 weight percent of trifunctional meta-glycidylamine; 5 to 15 weight percent of tetrafunctional para-glycidyl amine 3to 10 weight percent of polyamide 6 particles; 3 to 10 weight percent ofpolyamide 6/12 copolymer particles; 10 to 20 weight percent ofpolyethersulfone; and 15 to 25 weight percent of4,4-diaminodiphenylsulfone.
 18. A composite part that comprises apre-impregnated composite material according to claim 1 wherein saidmatrix has been cured at said curing temperature.
 19. A method formaking a pre-impregnated composite material for curing at a curingtemperature, said method comprising the steps of: A) providing areinforcing fiber; and B) impregnating said reinforcing fiber with amatrix wherein said matrix comprises: a) a resin component comprising adifunctional epoxy resin and at least one multifunctional aromatic epoxyresin with a functionality greater than two; b) a thermoplastic particlecomponent comprising high melting thermoplastic particles that have amelting point above said curing temperature and low meltingthermoplastic particles that have a melting point at or below saidcuring temperature; d) a thermoplastic toughening agent; and e) a curingagent.
 20. A method for making a composite part comprising the step ofcuring a pre-impregnated composite material according to claim 1 at saidcuring temperature.